A 911 call made in the minutes before George Zimmerman shot Trayvon Martin could turn out to be one of the most crucial pieces of evidence in the case. In the background of the audio recording, you can hear someone screaming for help right before the shot that killed Martin rings out.

Zimmerman has been charged with second-degree murder in Martin's death. He says he shot the teen in self-defense.

If the voice on that call is Zimmerman’s, then it could help back up his story, showing that he was under attack. If it's Martin's, then it could show Zimmerman was acting in an aggressive way.

So who is it and what exactly are they saying?

Voice identification experts hired by the prosecution say they don’t know for sure.

They compared the audio in the background of that 911 call to other voice samples including a 911 call made earlier by Zimmerman and audio of Martin recorded on his cell phone.

Senior analysts for the Forensic Communication Association (FCA) say there were “no robust matches,” but there were "several rather strong tendencies.”

They split up the audio into 16 different pieces and only found six utterances such as “wow,” “ow,” etc. that they say are potentially useful. The tidbits add up to 8 total seconds, but they need 10 seconds to meet the minimum standard.

Forensic acoustics consultant Alan R. Reich also listened to the voices on the call. He says the final word heard before the gunshot is actually “stop,” not “help.” He attributes it to Martin “tentatively” because he thinks it was said by the younger of the two speakers. He says his analysis is still ongoing.

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